Think Python - Denison University

19.3. Canvas widgets 183Exercise 19.1 Write a program that creates a GUI with a single button. When the button is presseditshouldcreateasecondbutton. Whenthatbuttonispressed,itshouldcreatealabelthatsays,“Nicejob!”.What happens if you press the buttons more than once? You can see my solution at thinkpython.com/code/button_demo.py19.3 Canvas widgetsOne of the most versatile widgets is the Canvas, which creates a region for drawing lines, circlesand other shapes. Ifyou didExercise 15.4 you arealready familiarwithcanvases.The methodcacreates anew Canvas:canvas = g.ca(width=500, height=500)widthandheightarethedimensions of the canvas inpixels.After you create a widget, you can still change the values of the options with the config method.For example, thebgoption changes thebackground color:canvas.config(bg='white')The value of bg is a string that names a color. The set of legal color names is different for differentimplementations of Python, but all implementations provide at least:white blackred green bluecyan yellow magentaShapes on a Canvas are called items. For example, the Canvas method circle draws (you guessedit)acircle:item = canvas.circle([0,0], 100, fill='red')Thefirstargumentisacoordinatepairthatspecifiesthecenterofthecircle;thesecondistheradius.Gui.py provides a standard Cartesian coordinate system with the origin at the center of the Canvasand the positive y axis pointing up. This is different from some other graphics systems where theoriginisinthe upper leftcorner, withthe yaxis pointing down.Thefilloption specifies that the circleshould be filledinwithred.ThereturnvaluefromcircleisanItemobjectthatprovidesmethodsformodifyingtheitemonthecanvas. For example, you can useconfigtochange any of thecircle’s options:item.config(fill='yellow', outline='orange', width=10)widthis thethickness of theoutline inpixels;outlineisthecolor.Exercise 19.2 Write a program that creates a Canvas and a Button. When the user presses theButton, itshould draw acircleon thecanvas.